The detection of weapons concealed underneath a person's clothing is an important obstacle to the improvement of the security of the general public as well as the safety of public assets like airports and buildings. Manual screening procedures for detecting concealed weapons such as handguns, knives, and explosives are common in controlled access settings like airports, entrances to sensitive buildings, and public events. It is desirable sometimes to be able to detect concealed weapons from a standoff distance, especially when it is impossible to arrange the flow of people through a controlled procedure.
One prior art approach utilizes a magnetometer to detect certain metallic objects. Unfortunately, this approach does not detect most organic polymer and composite materials that may be used to fabricate firearms, explosives, and other objects, which are frequently the subject of security inspections. In another approach, millimeter wave electromagnetic radiation is applied to provide images that can reveal objects concealed by clothing. This approach typically depends on the ability of a human inspector to visually detect one or more suspect objects from the resulting image. Accordingly, there are intrinsic speed limitations in these approaches, and such approaches are subject to variation with the ability of different inspectors.
Terahertz imaging is becoming more viable for many applications due to advances in detector and emitter technologies. One of the applications for THz imaging is the detection and identification of concealed weapons (e.g., in airport security screening lines). THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation, which poses no health risk for scanning of people. The target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds. Using THz radiation it is therefore possible to in principle detect explosives and biological weapons even if they are concealed in clothing, sealed packages, suitcases, etc since the THz radiation is readily transmitted through plastics, clothing, luggage, paper products, walls, and other non-conductive (non-metallic) materials.
Most THz imaging systems proposed in the past have been based upon a single THz source and detector pair that are scanned across the object space to be imaged. These systems consequently take a significant amount of time (typically minutes) to acquire the data to generate a THz image of even a single small object (e.g. of approximately a few square centimeters), and are not suitable to real-time acquisition of THz images. Another prior art approach utilizes a single THz array detector that detects the THz energy that passes through or reflects from the object, and reaches its collecting area over time. This imaging system requires a large area having good spatial details (resolution) in order to obtain an image of a large array of detectors. Similarly, to obtain good spatial resolution with conventional THz systems, the THz beam must have a small beam size, which reduces the area that can be imaged.
Based on the foregoing difficulties, it is apparent that there is a need for improved systems and methods for imaging of terahertz scenes using conventional optics and detectors to detect concealed weapons and explosives. It is also desirable to form THz images without a THz arrayed detector, which enables images to be orders of sharper magnitude.